It's probably very difficult to sell it as a discount. That implies 20% less work and the economics don't add up for that. That's 20% more people they have to hire, more managers, more complexity. Startups need to get things done faster because they have other fixed costs, and a trillion dollar company doesn't mind paying a little extra.
The nature of the work also means that people who put in the hours need to work less. They've made the mistakes, they make more accurate estimates, they know which theories don't work, they can debug more instinctively. They also have more battle tested code, simpler code, templates, hotkeys and macros. Managers are happy to hire someone who gets the sprint done by Wednesday then slacks off - but that can't be part of the contract, and it often involves hiring someone who has put in the hours.
I think the best way to go about it is agreeing to complete as much work, and arguing that 4 day weeks are the most efficient way to do this. Basically the industry tries to hire a senior who does as much work as 5 juniors for twice the salary, so this isn't an unrealistic idea.